In Britain, deficit denial is a charge the right lays at the door of the left. Across the Atlantic, the term is being hurled by ratings agencies at Washington's newly divided government. Markets got the jitters on Monday after Standard & Poor's said, for the first time in 70 years of US bond watching, that it could soon cease to regard the sovereign IOUs of the world's sole superpower as copper-bottomed guarantees.

The gnomes of the ratings agencies have had a dire crisis. Long in hock to vested interests, they only saw the private crunch coming after it arrived. There is justified anger as they now sit in judgment over small and cash-strapped democracies. The US, however, is a different matter. The planet regards its bonds as the safest of financial harbours, and no second-rater in any agency could transform this perception on his own. S&P's verdict hit home because it reflects what any Washington watcher can see. The US has grown keener on spending than paying its taxes, and is saddled with a political economy that punishes leaders who try to bring the two things back into line. The point here is not about the right deficit for this year or next, or the valuable role of pump-priming in a slump. It is rather about the need for a semblance of balance over the decades ahead.

The irony is that the starting point is not dire. The lapsing of Bush-era tax cuts, which were legislated to be temporary, and the rise in taxable incomes once prosperity returns would have done much of the work automatically, if they had been allowed to take their course. Instead, the debate has been framed by Paul Ryan, Republican chair of the House budget committee, who is bent on pushing all of the pain on to the expenditure side while actually cutting taxes. This would be fantastically hard in any ageing society, since the only pressure on the pensions bill is upward. It is doubly hard to achieve in the US, which is obliged to foot the runaway costs of marketised healthcare for the elderly. Without change, that burden would more than double as a share of GDP.

Mr Ryan's solution is to shred the medical safety net for the old. The social effects would be dire, and potentially compounded by derailment of Barack Obama's reforms that would put a lid on the costs. What matters for the deficit, though, is whether it is politically sellable. In a nation with the patchiest and priciest medicines in the rich world, that seems most unlikely.

If a failure to face up to the need to raise taxes is coupled to a failure to devise credible savings, the US will sleepwalk into a spiral in which debt interest gobbles up ever more of its resources. This disaster could still be averted, but with a check or balance in the way of every tax rise, it can no longer be ruled out.